I am trying to set my i3 configuration file in order to autostart calcurse when I log in. I have added the line
exec --no-startup-id calcurse
but it doesn't work.
If you want calcurse in an open window on startup, try exec --no-startup-id [xterm/urxvt/whatever_terminal_you_use] -e calcurse. (most terminal use -e to execute a command, check the terminal help if it does not work)
If you want calcurse to run in the background (to get event notifications, and you will still be able to access the interface by executing calcurse in a terminal), go with exec --no-startup-id calcurse --daemon